UCX osc: missing progress in shared memory

[devreal]

I encountered the following problem when forcing UCX with --mca osc ucx on a shared memory system (mainly for testing; I know it's not optimal but it shouldn't hang):

MPI_Win win;
MPI_Win_allocate(..., &win);
if (myrank == 1) {
  MPI_Win_lock_all(win); // <- hangs
}
MPI_Barrier(MPI_COMM_WORLD);

Process 1 will hang in MPI_Win_lock_all because MPI_Barrier will not progress UCX (presumably because UCX is not used for collectives in shared memory?). The UCX osc's progress callback gets called in the barrier call but since there are no active workers in process 0 it will not progress the operations required by MPI_Win_lock_all on process 1.

I have a possible fix that I will PR soon.